A spark plug is attached to a combustion apparatus such as an internal combustion engine, and is used to ignite an air-fuel mixture within a combustion chamber. In general, such a spark plug includes an insulator having an axial hole extending in an axial direction, a center electrode inserted into the axial hole, and a tubular metallic shell attached to the outer circumference of the insulator. The metallic shell has, on its outer circumferential surface, a thread portion which is screwed into an attachment hole of the combustion apparatus, a cylindrical tubular thread neck portion extending rearward from the rear end of the thread portion, and a seat portion which is provided on the rear end side of the thread neck portion and which comes into contact with the combustion apparatus indirectly via a ring-shaped gasket (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2008-108478).
In general, the metallic shell and the insulator are fixed together by means of crimping before the gasket is provided. More specifically, in a state in which the insulator is inserted into the metallic shell, a front end portion of the metallic shell is first inserted into an insertion hole of a receiving die, whereby the metallic shell is held by the receiving die. At that time, the seat portion comes into surface contact with an area around an opening portion of the insertion hole. Subsequently, with an annular pressing die, a load along the axial direction is applied to a rear-end-side opening portion of the metallic shell. As a result, the rear-end-side opening portion of the metallic shell is bent radially inward so as to form a crimped portion, which is engaged with a large diameter portion of the insulator, which portion swells radially outward. As a result, the metallic shell and the insulator are fixed together.
Incidentally, in order to realize a further improvement in airtightness, there has been proposed a method of bringing the seat portion into direct and close contact with a combustion apparatus without provision of a gasket (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-118659). That is, the seat portion is tapered off axially frontward, and the tapered seat portion is brought into surface contact with the combustion apparatus, whereby the sealing performance of the seal portion against the combustion apparatus can be enhanced, and, as a result, airtightness can be improved further.
However, in the case where the seat portion is tapered off frontward, when a load is applied to the metallic shell for crimp fixing, the material of the seat portion may deform radially inward along a contact surface of the receiving die. Therefore, the material of the seat portion may partially enter the insertion hole of the receiving die, resulting in biting of the metallic shell against the receiving die. If biting of the metallic shell occurs, extra labor is needed to remove the metallic shell from the receiving die, which may lower productivity. Furthermore, as a result of partial entry of the material of the seat portion into the insertion hole, a mark is formed on the seat portion, which may lower yield.
The present invention has been accomplished in view of the foregoing. An advantage of the present invention is a method of manufacturing a spark plug which can effectively suppress biting of a metallic shell against a receiving die, which would otherwise occur when the metallic shell and an insulator are fixed together, to thereby improve productivity, etc.